I have to admit that my experiences are very different. A little background, I have over 10,000 laps at my local track, where I was an instructor (which closed last summer ). I have done probably 600 laps in an NA M3 and probably 1100 laps in a SC E90 (575 and 625 kits) on this track. I never had any overheating issues, although our ambient temps way up North are lower than California.

I have been able to do much better laps with the SC. I saw appeciably higher speeds on both the front and back straight with front straight speeds increasing by 30kph on the front straight. The only place I saw any slower times was on the entry to the front straight that was a 2nd gear turn, so traction is an issue here with street tires. On r-compounds this was not a problem, so beyond requiring early braking points I didn't see any deterioration of lap times.